The genetic characterization of the chromosomal region within the H-2 gene complex controlling immune responses has been carried out quite a bit further during the past year. Considerable attention was paid to the regions I-E and I-C to determine that: 1) crossing over in H-2a haplotype occurred between those two regions; 2) I-E subregion controls genes with expression predominantly on B cells while I-C subregion controls genes with expression predominantly on the T cells; and 3) sequential immunoprecipitation showed that most B cell antigens to the right of the I-A subregion are expressed on molecules coded by I-E subregion genes. Tissue distribution of Ia antigens was carried out further by checking non-lymphoid cells such as epidermal cells and peritoneal neutrophils which suggest that I region gene products might have a role in tissues other than lymphoid origin. We have detected at least three new recombinants which have crossing over either within or adjacent to the I region which may be of value in future studies with Ir genes. We started studies on analyzing the different gene products associated with H-2 gene complex in several new haplotypes. In the past year we completed work on H-2p haplotype. A new technique was developed to produce heteroantisera to detect specific I region gene products in haplotypes where such antigens heretofore had not been identified. Finally, collaborative studies have shown functional value of genes coded within the I-C subregion in generating allosuppressive factors.